The present invention relates to an improvement in the method for the suspension polymerization of vinyl chloride monomer or a monomer mixture mainly consisting of vinyl chloride in an aqueous polymerization medium to produce a polyvinyl chloride resin. More particularly, the invention relates to an improvement in the method for the suspension polymerization of vinyl chloride monomer in a polymerization reactor system comprising an external heat exchanger connected to the reactor by a piping line in which the polymerization mixture is circulated from the bottom of the reactor to the upper part thereof through the said heat exchanger by means of a pump.
It is well known and generally understood that one of the important problems encountered in the suspension polymerization of vinyl chloride monomer in an aqueous medium contained in a polymerization reactor is the efficiency of heat exchange for quickly increasing the temperature of the polymerization mixture up to the polymerization temperature and efficient removal of the heat of polymerization in order to keep the temperature of the polymerization mixture within a narrow range during proceeding of the polymerization reaction. Namely, a conventional procedure for the suspension polymerization of vinyl chloride monomer is conducted by introducing water as the suspension medium, the monomer, polymerization initiator, suspending agent and other optional additives into a polymerization reactor equipped with a jacket for circulating hot water for heating of or cold water for heat removal from the polymerization mixture and a reflux condenser for heat removal. Once the temperature of the polymerization mixture has reached a specified polymerization temperature and the polymerization reaction is started, the temperature thereof is controlled at the specified polymerization temperature by removing the heat of polymerization by means of the jacket and the reflux condenser.
Needless to say, the productivity of the above described process depends on the length of time taken for completing a run of the polymerization reaction including the time for the preparatory step before the start of the polymerization reaction, the time for the polymerization reaction per se and the time for the processing of the polymerizate slurry into a powder product of the resin. The measure undertaken to decrease the time for the polymerization reaction per se is to increase the amount of the polymerization initiator or to use a polymerization initiator having high activity even when the polymerization-temperature is relatively low. Increase in the polymerization velocity by the above mentioned means is possible only with an improvement in the efficiency for the removal of the heat of polyymerization from the polymerization mixture during proceeding of the polymerization reaction.
It is a remarkable trend in recent years in such a process of polymerization that the process is conducted by using a polymerization reactor of a larger and larger capacity in order to enhance the productivity. An increase in the capacity of the reactor necessarily leads to a decrease in the area available for heat removal through the jacket relative to the volume of the reactor or the polymerization mixture. In order to compensate for the deficiency in the capacity of heat removal through the jacket, it is usual that a reflux condenser having a larger cooling capacity is installed to the reactor or, alternatively, the water circulated through the jacket is chilled by means of an additionally installed chilling means so as to increase the efficiency of heat removal.
The former means by increasing the capacity of the reflux condenser has a limitation due to a very difficult problem. When the rate of evaporation from the polymerization mixture and hence the rate of refluxing are increased by the increase in the capacity of the condenser, namely, violent foaming is unavoidably caused on the polymerization mixture eventually to carry over the liquid mixture so that heavy deposition of polymer scale takes place on all over the surface of the reactor and even inside of the reflux condenser resulting in a decrease in the cooling capacity or degradation of the product quality due to an increase in the amount of fish eyes. When such a large reflux condenser is brought into operation, in particular, at the beginning stage of the polymerization reaction, an adverse influence is caused on the particle size distribution of the resin product to contain coarser particles.
The latter means of chilling the cooling water is economically not practical due to the great costs required for installing and running the refrigerating machine. The efficiency of this means is still lower when the polymerization reaction is conducted for the production of a polyvinyl chloride resin having a relatively high average degree of polymerization for which the polymerization temperature must be decreased resulting in a decreased difference between the polymerization temperature and the temperature of the cooling water which can rarely be lower than 30.degree. C. when the cooling water is recirculated through the jacket.
As an alternative means of the above described prior art methods, a proposal has been made in Japanese Patent Kokai No. 54-24991, No. 56-47410 and No. 58-32606 and Japanese Patent Publication No. 64-11642 to provide an external heat exchanger or a cooling means which is connected to the reactor with a piping so as to circulate the polymerization mixture using a pump, typically, from the bottom of and to the upper part of the polymerization reactor through the heat exchanger where the polymerization mixture is cooled. The method using such a reactor system is indeed very effective if the matter concerned is merely to increase the cooling capacity. A serious problem unavoidable in this method, however, is the deposition of polymer scale on the inner walls of the circulation system of the polymerization mixture resulting in a decrease in the cooling capacity and adverse influences on the quality of the resin product. These drawbacks can be avoided by starting running of the external cooling system only at a moment when the monomer conversion into polymer has exceeded a certain limit or by using an external cooling system of a special type with large costs.